The present invention relates to a chip positive temperature coefficient (hereinafter, PTC) thermistor comprising conductive polymers having PTC properties. The present invention particularly relates to a laminated chip PTC thermistor.
PTC thermistors have been used as an overcurrent protection element. When an electric circuit gets overloaded, conductive polymers of a PTC thermistor, which have PTC properties, emit heat and thermally expand to become high resistance, thereby reducing the current in the circuit to a safe small current level.
The following is a description of a conventional laminated chip PTC thermistor (hereinafter, PTC thermistor).
The Japanese Patent Application Laid Open Publication No. H9-69416 discloses a structure of the conventional chip PTC thermistors. A conductive polymer sheet and an internal electrode of metal foil are alternately laminated so that number of the conductive polymer sheets is more than two, for providing a PTC thermistor element. Terminals coupled respectively with the opposing internal electrodes are provided on opposite side faces to complete a finished chip PTC thermistor.
FIG. 20 is a cross section of a conventional chip PTC thermistor. Referring to FIG. 20, a conductive polymer 1 is formed of polyethylene or the like high polymer sheet material mixed with carbon black or the like conductive particles and cross-linked. Internal electrode 2a, 2b, 2c, 2d made of a conductive material and a conductive polymer sheet 1 are laminated to form a PTC thermistor element 3. Provided on the side faces of the thermistor element 3 are terminals 4a and 4b, which are coupled respectively with the internal electrodes 2a, 2c and 2b, 2d. 
However, the above-described structure of conventional PTC thermistors exhibits following problems when they are intended to be made smaller in size, or capable of larger current.
In order to make a PTC thermistor to be compact and capable of handling a large current, the DC resistance of the PTC thermistor needs to be lowered. For reducing the specific resistance of the conductive polymer 1, it is effective to increase amount of the conductive particles contained in the conductive polymer. However, the increased conductive particles also effects a deterioration in the rising rate of the resistance, which being a key PTC characteristic, rendering it difficult to cut off the electric current when an abnormality happens.
The resistance can be lowered also by reducing the thickness of conductive polymer 1 placed among the internal electrodes 2a, 2b, 2c, 2d. However, this measure also leads to a deterioration in the rising rate of the resistance, like in the earlier example, and to a lowered withstanding voltage.
Furthermore, the resistance can be lowered also by increasing the opposing area of the internal electrodes 2a, 2b, 2c, 2d. The opposing area can be increased by increasing the number of laminated layers. However, the increased layers result in a greater thickness with a laminated body, which readily leads to a lower reliability in the connection between the internal electrodes 2a, 2b, 2c, 2d and the terminals 4a, 4b, being affected by a mechanical stress caused by expansion of the conductive polymer 1. Thus, there is a limitation in the increasing the number of layers.
Therefore, in order to lower the resistance, the effective opposing area per layer must be increased by making the distance between the internal electrodes 2a, 2b, 2c, 2d and the terminals 4a, 4b shorter. However, the portion of the conductive polymer 1 locating in the vicinity of the terminals 4a, 4b is physically restricted by the internal electrodes 2a, 2b, 2c, 2d, which means that it is not easy for the conductive polymer 1 to expand. As a result, when an overcurrent causes an expansion with the conductive polymer 1, the expansion remains small in the vicinity of the terminals 4a, 4b, leaving the specific resistance in the region to be small as compared with that in other regions. So, the rising rate of the resistance is impaired with a PTC thermistor whose distance between the internal electrodes 2a, 2b, 2c, 2d and the terminals 4a, 4b is short. Thus, the PTC thermistors had a problem that there is a possibility for the rising rate of the resistance to become low, if lowering of the resistance is intended to be realized through introduction of a laminated structure and increase in the effective opposing area.
The present invention addresses the above drawbacks, and aims to provide a chip PTC thermistor that is compact in shape, yet it is usable in the large current applications with a sufficient rising rate in the resistance.
A chip PTC thermistor of the present invention comprises:
a) a conductive polymer having PTC properties;
b) a first outer electrode in contact with the conductive polymer;
c) a second outer electrode sandwiching the conductive polymer with the first outer electrode;
d) one or more inner electrode disposed in between and parallel to the first and second outer electrodes and sandwiched with the conductive polymer;
e) a first electrode electrically directly coupled with the first outer electrode; and
f) a second electrode disposed electrically independently from the first electrode.
Where; when counting from one inner electrode, which is the closest to the first outer electrode, an inner electrode in the xe2x80x9cnxe2x80x9dth position is called as the xe2x80x9cnxe2x80x9dth inner electrode. If xe2x80x9cnxe2x80x9d is an odd-number, the inner electrodes are directly coupled with the second electrode; whereas, if xe2x80x9cnxe2x80x9d is an even-number, the inner electrodes are directly coupled with the first electrode. When the total number of the inner electrodes is an odd number, the second outer electrode is electrically directly coupled with the first electrode; whereas, if the total number of the inner electrodes is an even number, the second outer electrode is electrically directly coupled with the second electrode.
In the above PTC thermistor, distance from the odd-numbered inner electrode to the first electrode, or that from the even-numbered inner electrode to the second electrode, is defined as xe2x80x9caxe2x80x9d,
while distance among the adjacent inner electrodes, or distance from an inner electrode, locating next to the first outer electrode or the second outer electrode, to the first outer electrode, or the second outer electrode, is defined as xe2x80x9ctxe2x80x9d,
xe2x80x9caxe2x80x9d and xe2x80x9ctxe2x80x9d satisfy a relation of a/t=3-6.
In accordance with a structure that meets the above-described requirement, resistance of a PCI thermistor can be maintained low, and, at the same time, the rising rate of the resistance can be made sufficiently high. Thus the PCT thermistors of the present invention can be used for large current applications despite their compact size, and provide a sufficient capability for preventing an overcurrent. The terminology, xe2x80x9cthe rising rate of the resistance xe2x80x9d, used here with a PTC thermistor is defined as a ratio of resistance at an overcurrent divided by resistance at a normal current. The PTC thermistors in accordance with the present invention obtains the above-described functions and capabilities by controlling the parameters to be a/t=3-6.